This paper reports on experimental results of the photosensitized mechanism of HPD in cancer therapy. Human stomach cancer cells and Hela cells were chosen as the object of study. Two kinds of HPD were used being irradiated by either pulsed or CW laser light. The results showed that the main mechanism in photosensitized cancer therapy was singlet oxygen. The energy levels of HPD and HP were calculated and the photodynamic reaction process as well as it's corresponding physical model were discussed.